Frequency modulator



June 1944- G. L. USSELMAN ,351, 6

FREQUENCY MODULATOR Filed May 20, 1942 5 Sheets-Sheet 1 T132. I If? v OUTPUT INVENTOR, i Qsv/ E L [/5554 MAN.

ATTORNEY June 13, 1944.

G. L. USSELMAN 2,351,463

FREQUENCY MODULATOR Filed May 20, 1942 5 Shgets-Sheet 2 INVENTOR 550E515 L [/GSELMA/V ATTORNEY June 13, 1944. G L, US ELMAN 2,351,463

FREQUENCY MODULATOR ATTORNEY Patented June 13, 1944 umreo STATES PATENT orrlce 2,351,463 rarmmncr monum'roa George L. Usselman, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 20, 1942, Serial No. 443,699

The present invention relates to phase and frequency modulation and particularly to a new and improved wave lengthmodulator.

This type of frequency modulator like some other types uses a crystal oscillator with a tank circuit and two differentially modulated tubes but the method of and means for coupling the modulated tubes to the tank circuit and for oblength modulator in its simplest form as illustrated in Fig. 3 consists of an oscillation generator comprising a tube VI having grids 2 and 4 coupled to a crystal 6 and cooperating with the cathode to produce sustained oscillations-of a frequency determined primarily by v crystal 6. The grid 4 is supplied with positive direct current by resistor R2 the-end of which remote from grid 4 is connected by a high frequency bypass condenser 8 to the cathode III and ground. The grid 2 is connected to the cathode I and to ground by a grid biasing resistor RI. Oscillations are generated by virtue of the connections of grids 2 and 4 to opposed potential points at theterminals of crystal 6 and the connection of the cathode III to a point on a circuit RI, 8 and R2 efiectively in shunt to the crystal 6.

The anode I4 of tube VI is electronically coupled to the generating electrodes and the gen erated oscillations appear in the tuned tank circuit L|--CI coupled to the anode I4. A' direct current potential is supplied to the anode I4 via inductance LI and the direct current source is shunted by radio frequency bypass condenser I6.

The control grids 20 and 22 of modulator tubes V2 and V3 are connected by blocking condensers 24 and 26 to opposite ends of circuit L2C2. The cathodes 25 and 2! are also connected to the grids 20 and 22 by modulation potential blocking condensers 28 and 30 and bias resistors R3 and R4. The modulation potential source A is connected by bias resistors R3 and R4 and transformer 32 to the grids 20 and 22. Source 34 supplies a negative bias to grids 20 and 22. The center point of coil L2 of tank circuit L2-C2 is directly connected by lead I to a point on- LI. This feeds; generated voltage in phase to the grids 20 and 22. The inductances LI and L2 are coupled so that generated voltages are induced from Ll to L2 being shifted about and fed in phase opposition to the control grids 20 and 22.

The anodes 38 and 40 are connected to the 'ends of a tank circuit C3L3 with means for deriving the modulated output from this circuit. This circuit may be coupled to an aerial or a line directly or through amplifiers and frequency multipliers. The anodes of both tubes V2 and V3 may be connected to one end of the circuit L3C3, if desired, in which case switch P is connected to line Q. Other types of output couplings may be used.

In Fig. 2 regeneration is provided in the crystal controlled oscillator by a coupling condenser C3 between the anode I4 and control grid 2.

The crystal G'is between the control grid 2 and cathode III. In this generator the anode is no longer coupled to the generator substantially solely by the electron stream in the tube. The anodes 38 and 40 are tied together and con- .nected by line 46 to the lower end of circuit LIC.

Since the generated voltage fed back to the tank circuit LI-CI is fed to the end remote from the anode I4, the voltage is somewhat regenerative and helps in the oscillation generation operation and also permits production of frequency modulation in the system. The direct current potential for anode I8 is here fed to a point on LI. The output is taken from the tank circuit LICI by blocking condensers 48 and 50. The arrangement here is otherwise substantially Sim-'- ilar to the arrangement of Fig. 3. In Fig. 1 the oscillator is again as in Fig. 3. The anodes 38 and 40 of tubes V2 and V3 are tied together and coupled by line 54 to the anode end of tank circuit LI-C I Since the amplified generated wave energy is fed back to the anode end of LI-CI the generator is somewhat degenerative in action because 'the feed-back voltage opposes to some extent the generated voltage in LICI For proper operation of these frequency modulators both tank circuits LI-CI and L2-C2 must be tuned to resonance at the operating carrier frequency; that is, the frequency of the wave energy generated in the oscillation generator or a harmonic thereof where the generator serves also as a frequency multiplier. In Fig. 3 the tank circuit L3-C3 may be tuned to a harmonic of the generated frequency. The coupling H between tank circuits LICI and L2--C2 should be properly set for best results as may be seen from later description;

In describing the operation of my system the simplified arrangement of Fig. 3 will be used at this point.

Reference to Fig. 4 isadvantageous at this point. Fig. 4 is a 'vector diagram representing the plate and grid carrier voltage relations in the grid circuits of tubes V2 and V3 in this type of frequency modulator circuit.

In Fig. 4 vector A represents the voltage between the cathodes of tubes V2 and V3 and the centerpoint ll of inductance L2. Note that this voltage is in phase on the grids 20 and 22 and may be considered the voltage supplied conductively by lead [5. Vector C represents the induced voltage appearing between and the upper end (if L2 and on one of the control grids,

say 23, of tube V2. Vector D represents the induced voltage in the lower half of L2 and on the control grid of the other tube, say grid 22, of tube V3.

By using the proper coupling at H, the voltage induced in inductance L2 is rotated or shifted about 90 with respect to the-voltage in Ll. This voltage EgV2 of tube V2 is the limiting excitation voltage phase displacement in one direction.

The vector F representing the resultant grid voltage EgV2 of tube V3 is the limiting excitation voltage Phase displacement in the other direction.

The voltages represented by vectors 0 and D have 180 relation to each other because they are derived. from opposite ends of the tank circuit L2C2. Vectors C and D also have 90 relation to vector A due to the coupling relations of coils 40 Li and L2 in tank circuits L|Cl and L2-C2. This latter relation requires that both tank circuits be tunedfor the same carrier frequency and that the proper amount of inductive coupling be obtained. This follows from the fact that a 90? phase diflerence exists between the voltages of two loosely coupled tuned circuits each adjusted for resonance at the same frequency and adjusted for the proper amount of inductive coupling.

The blocking condensers-24 and 28 between circuit L2C2 and grids 2|! and 22 are large, and of small impedance so that they have no part in the phase shift of the grid voltages described above. As a consequence, the bias and modulator resistance R3 and RI play no part in the phase shift.

In the absence of modulation both tubes V2 and V3 amplify carrier energy (EgV2 and EgV3) and supply the same in equal amounts to circuit L3-C3. The energy therein takes up a mean phase which might well be also represented by vector A of Fig. 4.

When modulation is applied differentially to the control grids 20 and 22, one tube supplies more energy to the circuit C3L3 and thetotal energy therein takes up a phase more nearly like the phase of the energy supplied by that tube supplying the most of the energy to the tank circuit. Any amplitude modulation produced during this phase modulation process is balanced out since the-grid control is differential.

The total phase swing is limited by the phase displacement of the resultants represented by E and F and the-resultant energy is purely phase modulated. V

The amount of phase deviation is limited by the 75 v essentially similar to the arrangement of Fig. 3

except in the following important respects:

In Fig. 2 a feed-back condenser 03 couples the anode I to the crystal 6. This causes the system of Fig. 2 to operate more nearly as a frequency modulator, whereas the arrangement of Fig. 1,, which lacked this feed-back, produces substantially pure phase modulation.

In Fig. 2 the output is derived by lines including coupling condenser 43 and 50 from. the inductan'ce Ll in the generator tube VI anode circuit. The anodes 38 and 43 are tied together and connected by a feed-back lead 43 to the lower end of inductance Ll. The action of tubes V2 and V3 due to this coupling is regenerative with respect to the oscillator circuit'because of the anode connections of tubes V2 and V3 and the center tapped connection of grid coil L2 on the tank coil Ll of tube VI. This boosts the oscillator voltage.

The relation of the voltages set up in the input circuits of tubes V2 and V3 of Fig. 2, which-needs consideration, is shown in the vector diagram of Fig. 5a. Vector A represents the voltage EPVI, the voltage on the anode of tube VI. As in Fig. 4 this vector also in a sense represents the voltage between the center point a of inductance L2 and the cathodes of tubes V2 and V3and is the excitation voltage which reaches grids 23 and 22 in like phase.

The vector B represents the voltage EPV2--V3 on the anodes of tubes V2 and V3. Note that this voltage is reversed with respect to the voltage represented by vector A due to the action of tubes V2 and V3 and the fact that this voltage is fed by line 43 to the lower end of inductance Ll which is radio frequency grounded at its center. Vector Bl represents the voltage B reversed; that is, vector Bl represents the voltage set up at the anode end of inductance Ll by the preceding voltage represented by vector B. Note here that BI is now in phasewith vector A to produce regeneration and that during operation this voltage and the voltage represented by B swings in phase as shown by the dotted lines but aids the voltage represented by vector A thereby regenerating the voltages.

Vector C represents the voltage induced across the upper half of coil L2 from LI and vector D represents the voltage induced across the lower half of coil L2 from Ll. These voltages representis the generated frequency or a harmonic thereof) and the coupling adjustment they are obposed and substantially in quadrature with respect to the voltage represented by A as in Fig. 4.

Vector E represents the resultant voltage EgV2 on th grid 23 of tube V2, while vector F represents the resultant voltage EgV3 on the grid 22 of the v3.

In Fig. 1 as in Fig. 3, there-is no regeneration in the crystal oscillator including tube VI and this system operates more nearly as a phase modulator since very little feed-back reaches the oscillator. Moreover, in Fig. 1 the voltage fed back by line Bl goes to the top of inductance LI and the action is degenerative because the grids 20 and 22 and anodes I8 and In of tubes V2 and V3 are coupled back to the anode ll end of inductance LI and the voltage fed back bucks the'generated voltage.

The voltage vectors of the system in Fig. 1 are shown in Fig. 6. The vectors are related substantially as in Fig. ,5a and the vector diagram of Fig. 6 is believed substantially self-explanatory. In the system of Fig. l as illustrated vectorially, the regeneration voltage represented by vector Bl in Fig. a is no longer present. The vector B representing the voltage fed back by lead 54 opposes the voltage represented by vector A, thereby indicating the degenerative action. This action should not be carried to an extent such as would reduce the output of the system too great an amount.

In Fig.6 I have added two vectors, RI and R2, which are the resultants of vectors A and B and these vectors RI and R2 represent the resultants of A and B in the extreme positions of modulation.

A vector R, which is a resultant of the vectors RI and R2, could be added'and could be said to represent the output voltage of the modulator as it swings between the limits RI and R2.

The arrangementof Fig. 2 may be modified as follows; The ilad l5 may connect the lower end of inductance Ll to the mid-point on inductance L2 and the lead 46 may connect the anodes 38 and 40 to the upper end of the inductance Ll. This embodiment-of my wave length modulator is shown in Fig. 2a of the drawings. In this arrangement as in Fig. 2, the condenser C3 provides regeneration between the tank Cl--Ll and the oscillator anode. 2; also the v feed-back by line 46 is regenerative as in Fig. 2

since it is connected to the anode end of the inductance LI, and lead I5 is connected to the lower end 01' Ll.

The operation of the embodiment illustrated in Fig. 2a is essentially as in the prior figures and a repetition of said operation at this point is believed unnecessary here. However, the vector diagram for Fig. 2a is shown in Fig. 5b. The following voltages are represented by the vectors in Fig. 5b:

Vector A represents the voltage of the upper end of coil Ll.

Vevtor A represents the voltage of the lower end oi coil Ll.

Vector C represents the voltage of the upper half of coil'L2.

Vector D represents the voltage of the lower half of coil L2.

Vector B represents the voltage of the anodes of tube -V2-V3.

Vectors E and F represent the voltageton the grids of tubes V2-V3.

As may be seen, vector A is vector A reversed because coil Ll is connected near the middle In Fig. 1 these vectors are substantially 'stationary during operation. In Fig. 2 the feedback voltage B operates to rotate the phases of all the vectors to some extent, thus resulting in some frequency modulation instead of pure phase modulation as in Fig. 3. However, even in the rotated position of the vectors E, F and A, they are related vectorially substantially as shown. The same is true of the system in Fig. 1 except that due to the absence of regeneration and use of degenerative action described above, less frequency modulation is produced.

I claim:

1. In a wave length modulation system, a regenerativ oscillation generator, a pair of electron discharge devices each having an electron emission electrode, an electron flow control electrade and an electron receiving electrode, a first coupling between said generator and said electron fiow control electrodes for applying voltages from the generator to the electron flow control electrodes of like phase, a. second coupling for impressing voltages from the generator to the electron flow control electrodes in opposed phase,

said last named voltages being substantially in phase quadrature with respect to the said first mentioned voltages'on the electron flow control electrodes, an output circuit coupled to said system, and means for differentially controlling the gain of said devices.

2. In a wave length modulation system, a regenerative oscillation generator, a pair of electron discharge devices each having an electron emission electrode, an electron flow control electrode and an electron receiving electrode, a conductive coupling between said generator and said electron flow control electrodes for applying voltages from the generator to the electron fiow' control electrodes of like phase, an inductive coupling for impressing voltages from the generator on to the electron flow control electrodes in opposed phase, said last named voltages being substantially in phase quadrature with respect to the first mentioned voltages on the electron flow control electrodes, an output circuit coupled to said electron receiving electrodes, and means f6r diflerentially controlling the gain of said devices.

3. In a wave length modulation system, a source of oscillations, a pair of electron discharge devices each having an electron emission electrode, an electron flow control electrode and an electron receiving electrode, a circuit interconnecting'the electron flow control electrodes of said devices, a first coupling between said source of oscillations and said circuit for applying inphase voltages from the source to the electron flow control electrodes, a second coupling between said' source of oscillations and said circuit for applying opposed voltages from the source tofthe electron flow control electrodes, said last named voltages being-substantially in'phase quadrature with respect to the first voltages on devices each having an electron emission electrode, an electron flow control electrode and an electron receiving electrode, a circuit interconnecting the electron flow control electrodes of said devices, a first coupling between said source of oscillations and said circuit for applying inphase voltages from the source to the electron flow control electrodes, a second coupling between said source of oscillations and said circuit for impressing opposed voltages from the source to the electron flow control electrodes, said last named voltages being substantially in phase quadrature with respect'to the first mentioned voltages on the electron flow control electrodes, 9, feed-back coupling between the electron receiving electrodes of said devices and said circuit. means for diflerentially controlling the gain of, said devices to'thereby correspondingly vary the length of the oscillations generated, and an output circuit coupled to said system.

5. In a wave length modulation system, a regenerative oscillation generator, a pair of electron discharge devices each having an electron emission electrode, an electron flow control electrode and an electron receiving electrode, a circuit tuned to the desired operating frequency coupling said flow control electrodes in push pull relation, a first coupling between said oscillation I generator and said tuned circuit for app y g inphase voltages from the generator to the 'electron fiow controLelectrodes, a second'coupling between said oscillationgenerator and said tuned circuit for impressing opposed voltages to the eelctron flow control electrodes, said last named voltages being substantially in phase quadrature trolling the gain of said devices at signal frequency.

8. In a wave length modulation system, a generator including a circuit having inductance tuned to the, desired operating frequency, a pair 4 of electron discharge devices each having an electrol electrodes, a loose coupling between said inductances for impressing opposed voltages from the generator on the electron flow control elecwith respect to' the said first voltages on the electron fiow control electrodes, an output circuit coupled to said electron receiving electrodes, and

means for diflerentially controlling the gain of said devices at signal frqeuency.

6. In a wave length modulation system, a regenerative oscillation generator, a pair of. electron'discharge devices each having an electron emission electrode, an electron flow control electrode and an electron receiving electrode, a circuit tuned to the desired operating frequency coupling said flow control electrodes in push-pull relation, a first coupling between said generator and said tuned circuit for applying in-phase voltages from the generator to the electron flow control electrodes, a scond'coupling between said generator and said tuned circuit for impressing opposed voltages to the electron flow control electrodes, said last named voltages being substantially in phase quadrature with respect to the first mention d voltages on the electron fiow'control electrodes, a degenerative feed-back circuit coupling the electron receiving electrodes of said devices to said generator, an output circfuit cou-' pled to said electron receiving electrodes; and means for differentially controlling the gain of said devices at signal frequency.

7. In a wave length modulation system, a generator including a circuit having inductance tuned to the desired operating frequency, a pair of electron discharge devices each having an electron emission electrode, an electron flow control electrode and an electron receiving electrode, a circuit including inductance tuned to the desired operating irequency coupling said flow control electrodes 'in push-pull relation, a first coupling between said circuits for applying in-phase voltage from. the generator to theelectron flow control electrodes, a loose coupling between said inductances for impressing opposed voltages from the generator on the electron flow control electrodes said last named voltages being substantially in phase quadrature with respect to the said first voltages 0n the -electron' flow voltages on the electron receiving electrodes, a

regenerative feed-back coupling between said electron receiving electrodes and said first named circuits, and means for diflerentially controlling the gain of said devices at signal frequency.

9. In a wave length modulation system, a first circuit comprising parallel inductance and capacity tuned to the frequencyof oscillatory energy to be wave length modulated, connections to said circuit for impressing oscillatory energy of said frequency thereon, a pair of electron discharge devices each having a control electrode, a cathode and an output electrode, a second circuit comprising parallel inductance and capacity tuned to the frequency of the oscillatory energy to be wave length modulated, a feedback connection coupling the output electrodes of said devices to said first parallel tuned circuit, couplings between said circuits and the control electrodes and cathodes of said devices including the mutual coupling between said inductances for impressing. voltages of the frequency of said oscillatory energy in substantially opposed phase relation on the control electrodes of said devices and for impressing other voltages of the same frequency andof substantially like phase on the control electrodes of said devices, said other voltages being impressed in phase displaced relation with respect to the substantially phase Op osed voltages on said control electrodes, a source of modulating potentials and connections between said source of modulating potentials and the control electrodes of said devices to vary the amplification of said devices in op- 10. In a wave length modulation system, a first circuit comprising parallel inductance and capacity tuned to the frequency of oscillatory energy to be wave length modulated, connections to said circuit for impressing oscillatory energy of said frequency thereon, a pair of electron dis charge devices each having a cathode and an output electrode, a second circuit comprising parallel inductance and capacity tuned to the frequency of the oscillatory energy to be wave length modulated, a feedback connection coupling the output electrodes of said devices together and to' said first parallel tuned circuit, couplings between said circuits and the control electrodes and cathodes of said devices including the mutual coupling .between said in in phase opposed relation-"on the control eleca control electrode,

trodes of said devices and for impressing other voltages of the same frequency and of substantially like phase on the control electrodes of said in opposed phase relation in accordance with the modulating potentials.

11. In a wave length modulation system, a first tuned circuit comprising parallel capacity and inductance tuned to the frequency of the oscillations to be wave length modulated, means for impressing oscillatory energy of said frequency on said tuned circuit, a second tuned circuit comprising parallel'inductance and capacity tuned to said frequency, there being coupling between the inductances of said circuits, a pair of electron discharge devices each having a control electrode, an output electrode and a cathode, connections coupling said second named tuned circuit between the control electrodes of said devices, a coupling between a point on said first named tuned circuit and a point intermediate the terminals of the inductance of said second tuned connections coupling said second named tuned circuit between the electrodes of saiddevices, a coupling between one end of the inductance of said first named tuned circuit and a point intermediate the terminals of the inductance of said second named tuned circuit, a connection coupling the anodes of said devices togetherand to the other end of said first named tuned circuit, a source of modulating potentials and connections for applying modulating potentials from said source to corresponding electrodes in said devices in phase opposed relation.

14. In a wave length modulation system, a source of oscillatory energy the wave length of which is to be modulated, a pair of electron discharge devices each having a control electrode,

a cathode and an output electrode, a circuit comand said circuit forimpressing voltages substancircuit, a connection coupling the anodes of said" tuned circuit comprising parallel capacity and inductance tuned to the frequency of the oscillations to be wave length modulated, means for impressing oscillatory energy of said frequency on first tuned circuit, a second tuned circuit comprising parallel inductance and capacity tuned to said frequency, there being coupling between the inductances of said circuits, a pair of electron discharge devices each having a control electrode, an output electrode and a cathode, connections coupling said second named tuned circuit between the control electrodes of said devices, a coupling between a point on said first named tuned circuit and a point intermediate the terminals of the inductance of said second named tuned circuit, a connection coupling the anodes of said devices together and to said point on said first named tuned circuit, a source of modulating potentials and connections for applying modulating potentials from said source to the control electrodes of said devices in phase opposed relation.

13. In a wave length modulation system, a tuned circuit comprising parallel capacity and inductance tuned to the frequency of the oscillations to be wave length modulated, means for impressing oscillatory energy of said frequency on said first tuned circuit, a second tuned circuit tially opposed in phase on the control electrodes of said devices, a coupling between said source and said control electrodes for impressing other voltages of substantially like phase on the control electrodes of said devices, said other voltages being displaced in phase with respect to said first voltages, a source of modulating potentials, connections between said source of modulating potentials and corresponding electrodes in said devices to vary the amplification of'said devices in opposed phase relation in accordance with the modulating potentials and an output circuit coupled to said system.

15. In a wave length modulation system, a circuit comprising parallel inductance and capacity tuned to the frequency of oscillatory energy to be modulated in wave length, connections to said circuit for impressing oscillatory energy thereon, a pair of tube devices each having electrodes between which'an electron discharge takes place when the tu-beelectrodes are energized, a second circuit comprising parallel inductance and capacity tuned to the frequency of the oscillatory energy to be wave length modulated, connections from said second circuit to corresponding electrodes of said discharge devices for exciting the 'same substantially in opposed phase relation,

couplings between said circuits including a connection between a point on said first named inductance and a point intermediate the terminalsof said second named inductance and the mutual coupling between said inductances for impressing voltages of substantially like phase by way of said second named circuit on the said corresponding electrodes of said devices and for impressing other voltages of substantially opposed phase on the said corresponding electrodes of said devices, a source of modulating potentials, connections from said source of modulating potentials to corresponding electrodes in said comprising parallel inductance and capacity tuned to said frequency, there being coupling beelectrode. an output electrode and a cathode,

devices to vary the conduc'znce of said devices in opposed phase relation in accordance with the modulating potentials and an output circuit coupled to said system.

GEORGE L. USSELMAN} 

